More than 250 counter terrorism police are now involved in what the home secretary has described as a "major investigation".

Speaking after a meeting of the government's emergency committee, Cobra, Ms Rudd said the government was using enormous resources to try and identify those responsible for the attempted murder.

"I want to stress that they are proceeding with speed and professionalism," she said.

Image copyrightGetty ImagesImage caption
More than 250 counter terrorism police are involved in the investigation

She added: "This investigation is focused on making sure that we keep people safe and also that we collect all the evidence so that when it comes to attribution [of the attack] we will be absolutely clear where it should be.

"The police have said that if anybody thinks they have any additional information they would welcome them coming forward.

"There is also substantial amounts of CCTV they have to go through. This is a painstaking, detailed investigation and the police need to be given the space and time to get on with it."

Mr Skripal was convicted by the Russian government of passing secrets to MI6, but given refuge in the UK in 2010 as part of a "spy swap".

Russia has denied any involvement. UK Foreign Secretary Boris Johnson has said the country will respond "robustly" if Moscow is found to have been behind the incident.

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The challenge for MI6

BBC security correspondent Gordon Corera

Observers say that with Russia, it increasingly looks as if the intelligence agencies are happy with "implausible deniability" - even if it seems obvious they are behind an act, they simply say "prove it" and try to muddy the waters with misinformation as much as possible.

If the theory of Russian revenge and message sending is correct, it raises difficult questions for British intelligence.

MI6 relies on recruiting agents like Skripal in countries like Russia to provide secrets.

It has always prided itself on keeping the identity of those agents secret in order to protect them.

But if the perception emerges that it cannot protect those agents - even if they are in the UK - then it will make it much harder to do its job and recruit agents to gather intelligence.

Ms Petrova said Ms Skripal "never shared her problems" - even when her father was arrested for spying in 2004, with the incident broadcast on Russian state TV.

She said "everyone was shocked" at the news, adding that Ms Skripal, who was visiting her father from Moscow, was a "normal kind of person" who enjoyed spending time in the UK and even considered applying for citizenship.